<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <meta http-equiv="X-UA-Compatible" content="ie=edge">
        <title>Document</title>
        <script src="./lib/vue-2.4.0.js"></script>
    </head>
    
    <body>
        <div id="app">
            <input type="text" v-model="firstname"> + 
            <input type="text" v-model="lastname"> =  
            <input type="text" v-model="fullname">                 
        </div>

        <script>
            var vm = new Vue({
                el: '#app',
                data: {
                   firstname: '',
                   lastname: '',
                   fullname: ''
                },
                methods: {
                   
                },
                // 使用这个属性，可以监视 data 中指定数据的变化，然后出发这个 watch 中
                // function 处理函数
                watch: {
                    firstname: function(newVal, oldVal) {
                        // console.log("监视到了firstname的变化")
                        // this.fullname = this.firstname + "-" + this.lastname

                        console.log(newVal + " --- " + oldVal)
                        this.fullname = newVal + "-" + this.lastname
                    },
                    lastname: function(newVal) {
                        this.fullname = this.firstname + "-" + newVal
                    }
                }
            });
        </script>
    </body>
</html>